Changes in serum thyroid hormone levels and thyroid gland activity of artificially maturing female Japanese eel(Anguilla japonica)

In order to evaluate the involvement of the thyroid gland in reproduction,thyroid functioning was described in female cultivated Japanese eel(Anguilla japonica) in progressive stages of sexual maturation induced by chum salmon pituitary homogenate(SPH) treatment.Serum thyroid hormones,thyroxine(T₄) and triiodothyronine(T₃) were also measured in each stage.Thyroid gland activity(epithelial cell height) was high before SPH injection(previtellogenic stage),further increasing at the early vitellogenic stage,thereafter decreasing to late vitellogenic and migratory nucleus stages.The profiles of both T₃ and T₄ changed during vitellogenesis,being high during previtellogenic and early vitellogenic stages,and subsequently declining,thus mimicking thyroid gland activity.These results suggest that the thyroid has relation with eel ovarian development during artificial maturation.